Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing our new design.

The dotted and evenly-dashed broken lines, specifically, a herringbone patterned background, 24 circles of a 5-by-5 array of circles, an icon and text in the bottom left of the banner, and an outermost broken line illustrating a boundary of a display screen or a portion thereof, illustrate portions of the graphical user interface that do not constitute part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 